The iBiquity Digital Corporation HD Radio™ system is designed to permit a smooth evolution from current analog amplitude modulation (AM) and frequency modulation (FM) radio to a fully digital in-band on-channel (IBOC) system. This system delivers digital audio and data services to mobile, portable, and fixed receivers from terrestrial transmitters in the existing medium frequency (MF) and very high frequency (VHF) radio bands. Broadcasters may continue to transmit analog AM and FM simultaneously with the new, higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog to digital radio while maintaining their current frequency allocations.
Both the hybrid and all-digital formats of the HD Radio™ system utilize a plurality of orthogonal frequency division modulated subcarriers. Reference subcarriers are inserted among the data subcarriers. Coherent demodulation is used in the digital portion of an FM IBOC (In-Band On-Channel) signal for the HD Radio™ system. The multiple roles of the reference subcarriers for acquisition, tracking, estimation of channel state information (CSI) and coherent operation have been described in U.S. Pat. No. 6,549,544. This system was designed for operation in the FM broadcast band (88-108 MHz) with fading bandwidth to accommodate vehicles at highway speeds. The various coherent tracking parameters are estimated using filters with bandwidths that approximate the maximum expected Doppler bandwidth (roughly 13 Hz). With a fixed antenna, the pertinent tracking statistics of the input signal to the tracking algorithms are assumed to vary at a rate no greater than the Doppler bandwidth. However the use of switch diversity antennas in vehicle windows introduces abrupt transients in the coherent tracking which degrade performance of the coherent digital demodulation.
This invention provides improvements to the coherent tracking algorithms which are used autonomously with a blind switch diversity antenna system. These same improvements can also mitigate degradation due to impulsive noise or nongaussian noise such as from an adjacent FM analog interferer. In addition, performance of receivers including fast Automatic Gain Control (AGC) is improved.